Disease caused by the gram-positive bacterium, Staphylococcus aureus, represents a significant source of morbidity and mortality annually in the US. Particularly, the recent emergence of community-acquired methicillin-resistant S. aureus (CA-MRSA) is a source of great concern for public health officials. A distinct clone of CA-MRSA known as USA-300 is increasingly appreciated as one of the most dangerous emerging pathogens due to its hypervirulence and its propensity to spread through the general public. A better understanding of the underlying advantages inherent to CA-MRSA over traditional S. aureus isolates therefore pertains directly to global public health. Polyamines including putrescine, spermidine and spermine are reportedly synthesized by all forms of life. These polycationic compounds exert highly pleitropic effects on cellular physiology and their synthesis is essential in mammals. Increased polyamine production is observed in actively growing tissue including sites of inflammation and healing wounds. Polyamines also modulate numerous aspects of bacterial physiology and generally promote bacterial growth. However, none of the 13 sequenced S. aureus genomes encode canonical polyamine biosynthetic pathways distinguishing S. aureus from nearly all other living organisms. More importantly, exogenous spermine and spermidine exert bactericidal effects on S. aureus at concentrations that do not affect the growth of other organisms. The only notable exception is USA-300, which is not inhibited by spermine or spermidine. USA-300 harbors a unique genetic island known as ACME that is not present in other S. aureus lineages, including other CA-MRSA clones. In contrast to the polyamine resistance inherent to WT USA-300, the ?ACME mutant is susceptible to exogenous spermine and spermidine. This island contains a homologue of E. coli speG encoding a spermine/spermidine acetyl transferase (SSAT), which my laboratory has established is necessary and sufficient for ACME-associated polyamine resistance. A role for ACME in the hypervirulence of USA-300 S. aureus has been established;yet a mechanistic understanding of the advantage afforded CA-MRSA by ACME has not been developed. We propose that speG-mediated polyamine resistance may provide a novel explanation for the advantage conferred to USA-300 by ACME. Thus, this proposal will test our Central Hypotheses: 1) Host polyamine production is a novel innate immune effector specific for S. aureus and 2) The acquisition of ACME by S. aureus represents a highly significant evolutionary adaptation providing polyamine resistance and contributing to the emergence of the USA-300 epidemic. PUBLIC HEALTH RELEVANCE: Polyamines are compounds made by all living organisms. They serve many functions and are highly produced in rapidly growing cells. Interestingly, the human bacterial pathogen Staphylococcus aureus appears to be the only known organism that is incapable of synthesizing polyamines. Several complete genomes sequences are available for this bacterium, yet none contain canonical polyamine biosynthetic pathways. Moreover, polyamines at levels found in the human body are highly toxic to S. aureus. The only exception is a newly emerging clone of S. aureus known as USA-300 that causes highly invasive disease in patients with no obvious risk factors. This methicillin-resistant S. aureus (MRSA) strain has become a leading cause of morbidity and mortality associated with infectious disease in the US and worldwide. We have identified the gene in USA-300 that confers resistance to host polyamines. It is encoded on a genetic island present in USA- 300 isolates, but absent from most other strains of S. aureus. The goal of this proposal is to: 1. Show definitively that S. aureus does not make polyamines distinguishing it from nearly all other forms of life. 2. Understand how polyamines exert toxicity towards S. aureus. 3. Demonstrate that host polyamine production limits the proliferation of S. aureus during disease. 4. Show that the success of USA-300 MRSA stems, in part, from the polyamine-resistance associated with this strain. With this information, we can begin to test whether modulating host polyamine production prevents invasive disease caused by MRSA, a dangerous and costly human pathogen.